1. Field
The present invention is directed toward an optically bistable system and method that incorporates amorphous semiconductor material in an optical feedback cavity. The amorphous material may be thermally altered by electromagnetic radiation to alter the transmission or reflection of a probe beam.
2. State of the Art
There has been an "invasion" of optical devices and methods into traditional electronics-based communications and computing fields. This invasion has occurred in areas such as individual optical logic devices, interconnections in optical communications, and novel all-optical computer architecture. A device used in such applications is the optically bistable device. A system is said to be optically bistable when it can exhibit two different optical output signals for a single optical input signal.
Optical bistability occurs as a result of nonlinearity in light/matter interactions. Nonlinearity in the material can occur because of optical absorption by the material or a change in the refractive index. In absorptive optical bistability, an increase in input power may result in an increase or a decrease in absorption, i.e., the optical medium becomes less or more transparent as more light is shined onto it. Optical bistability is described in S. D. Smith, Applied Optics, infra, the disclosure of which is incorporated herein by this reference.
In crystalline semiconductors, optical bistability can be achieved by a thermal shift of the energy band gap of the material, which results in light being absorbed. A number of optically bistable devices which utilize crystalline semiconductor materials have been discussed in the literature. Three materials which have been investigated are: GaAs, InSb, and ZnSe. (S. D. Smith: "Optical Bistability, Photonic Logic and Optical Computation," Applied Optics. Vol. 25, No. 10, page 1550 (1986)). However, while these crystalline materials show large optical bistability, they are limited in the spectral range of electromagnetic radiation to which they will respond, the temperature at which they will properly function (particularly InSb), and feasibility in terms of large-scale production and use in practical applications.
In amorphous (conventionally designated "a-") semiconducting films, it has been postulated that electronic, thermal, and photostructural (laser-induced structural changes) act simultaneously to produce nonlinear behavior in some cases. (J. Hajto and I. Janossy, "Optical Bistability Observed in Amorphous Semiconductor Films," Philos. Mag., 47, 347 (1983)). In a-GeSe.sub.2 films, thermal effects have been considered as one of the causes for optical bistability in the Urbach region of the absorption edge, where the absorption coefficient .alpha. is temperature dependent. Id. The temperature dependence of the refractive index of hydrogenated amorphous silicon (a-Si:H) has been measured and the modulated thermoreflectance has been calculated. (M. H. Brodsky and P. A. Leary, J. Non-cryst. Solids 35 and 36, 487 (1980)).